Ethylene copolymers are widely used in various application fields, such as the production of films, blow-molded products, pipes and coating materials for electric transmission cables. With respect to any of these applications, it is required that an ethylene copolymer not only contain few impurities, such as wax, gels, but also exhibit excellent properties, such as high impact strength and high environmental stress cracking resistance (hereinafter, frequently referred to as “ESCR properties”). However attempts to vary the molecular structure of a polymer to cause an improvement in one such property often results in a loss of performance in another. For instance polymers exhibiting high stiffness and heat resistance should have high crystallinity and low comonomer content, however this can cause a loss of toughness, ESCR, low optical properties and poor heat seal performance. Similarly for improved polymer processability (low extrusion amp and back pressure and no melt fracture) it is desirable to use polymers having a low molecular weight, and a broad molecular weight distribution with significant levels of long chain branching. However broad molecular weight distribution, especially at low polymer molecular weight, often causes wax buildup on the die, smoke generation on the extruder, and taste and odor problems in the resulting fabricated articles.
It is known that improvement in impact and environmental stress crack resistance of an ethylene copolymer, can be achieved by decreasing the comonomer content of the low molecular weight fraction of the ethylene copolymer to a level as low as possible while increasing the comonomer content of the high molecular weight fraction of the ethylene copolymer to a level as high as possible. It has also been demonstrated (as for example by Zhou et al, Polymer, Vol. 24, p. 2520 (1993)) that large strain properties such as toughness, tear, impact and ESCR can also be improved by the presence of “tie molecules” in the resin. High molecular weight molecules with the highest comonomer content (that is the highest degree of short chain branching) are responsible for the formation of most of the tie molecules upon crystallization.
Thus it would be highly desirable for a copolymer to have a specific comonomer content distribution characteristic, wherein, in one aspect, the lower the molecular weight of a copolymer fraction in a molecular weight distribution of a said copolymer, the lower the comonomer content of the copolymer fraction; and, in the other aspect, the higher the molecular weight of a fraction of said copolymer, the higher the comonomer content of the copolymer fraction.
However, in ethylene copolymers which are produced using a conventional Ziegler-Natta catalyst, it is likely that the lower the molecular weight of a copolymer fraction, the higher its comonomer content. Thus, such conventional ethylene copolymers have a comonomer content distribution which is completely contrary to the above-mentioned desired comonomer content distribution. Therefore, such conventional ethylene copolymers are at a disadvantage with respect to desirable properties, such as improved impact strength and ESCR.
Attempts to maximize toughness, modulus, impact strength and ESCR of ethylene copolymers has resulted in the preparation and use of blend compositions made out of two or more ethylene copolymer components of differing molecular structures. In addition to separately blending selected individual polymer components after their manufacture and isolation (so called “off-line” blending), such compositions can also be prepared by a method in which a copolymerization of ethylene with a comonomer is conducted by a multi-stage polymerization, using a plurality of different polymerization reactors, capable of providing different copolymerization conditions. This allows so called “in reactor” or “in process” production of ethylene copolymers comprising a mixture of a low molecular weight copolymer component, having a low comonomer content, and a high molecular weight copolymer component having a high comonomer content.
Such blend compositions containing solely Ziegler catalyst products are described in a number of patents. For example, Nelson (U.S. Pat. No. 3,280,220, Phillips Petroleum) teaches that a blend of an ethylene homopolymer of low molecular weight (formed in a solution process) and an ethylene/butene-1 copolymer of high molecular weight (formed in a particle forming process) provides higher ESCR and is more advantageous for containers and pipe than other such blends.
Hoblitt et al. (U.S. Pat. No. 3,660,530, the Dow Chemical Company) teaches a method where part of a homopolymer produced after a first reaction step is subjected to 1-butene. The still active catalyst then produces a block copolymer of polyethylene and polymerized butene-1. Both components are then admixed. The resultant blend has improved ESCR properties.
Fukushima et al. (U.S. Pat. No. 4,438,238) disclose blends consisting of components with densities between 0.910 and 0.940 g/cm3 and broad molecular weight distributions with the polymers having substantially no long chain branches. These blends were found to have processability similar to that of high pressure polyethylene
Bailey et al. (U.S. Pat. No. 4,547,551) teach that ethylene polymer blends of a high molecular weight ethylene polymer, preferably an ethylene/α-olefin copolymer, and a low molecular weight ethylene polymer, preferably an ethylene homopolymer, both preferentially having a narrow molecular weight distribution and low levels of long chain branching, exhibit excellent film properties and a better balance of stiffness and impact and ESCR, than expected for polyethylene of comparable density and flow.
Morimoto et al. (U.S. Pat. Nos. 5,189,106, and 5,260,384) disclose blends consisting of a high molecular weight copolymer in combination with a low molecular weight homopolymer having good processability and excellent low temperature mechanical properties.
Boehm et al., (Advanced Materials 4 (1992) no 3, p 237), disclose the cascade polymerization process in which the comonomer is introduced in the high molecular weight fraction of the polymer resulting in a larger amount of comonomer being present at the same overall density. This in turn results in a polymer composition having improved rigidity-lifetime (failure time) compared to conventional unimodal copolymers. Several patents have also appeared teaching the process to produce such materials in such cascade processes including EP 0 022 376 (Morita et al).
Unexamined Japanese Patent Application Laid-Open Specification Nos. 61-221245 and 61-57638, disclose attempts to increase the comonomer content of high molecular weight copolymer fractions by a method in which a low molecular weight polymer having a low comonomer content and a high molecular weight polymer having a high comonomer content are separately produced and blended by means of a kneader, or a method in which a copolymerization of ethylene with a comonomer is conducted by multi-stage polymerization, thereby producing an ethylene copolymer comprising a mixture of a low molecular weight polymer component having a low comonomer content and a high molecular weight polymer component having a high comonomer content.
Finally, Sakurai et al (U.S. Pat. No. 4,230,831) disclose that it is beneficial to mix low density polyethylene with various blend compositions to improve polymer die swell or melt tension.
In single component ethylene copolymers produced by employing a Ziegler catalyst, some improvement is achieved with respect to impact resistance and ESCR properties. Such ethylene copolymers, however, inherently exhibit not only a broad molecular weight distribution but also a broad tail on both the low and high molecular weight side of the molecular weight distribution. The presence of the low molecular weight material can disadvantageously lead to wax formation. On the other hand, the high molecular weight material can disadvantageously lead to gel formation.
In addition, blend compositions which are a mixture of such ethylene copolymers produced by a Ziegler catalyst, may comprise component copolymers which are completely different from each other in properties, that is, a low molecular weight polymer component having a low comonomer content and a high molecular weight polymer component having a high comonomer content. This can lead to the component polymers undergoing phase separation such that the dispersion of the component polymers becomes non-uniform, and thus not only does the ethylene copolymer become non-uniform in properties but also gel formation occurs.
As an alternative to the use of Ziegler-Natta catalysts, the use of metallocene catalysts has recently been proposed (DE 31271332 ) and commercialized. As disclosed in, for example, Worldwide Metallocene Conference (Metcon) '93 May 26-28, Houston Tex., p. 171-172 and p. 235-244 (1993) and Proceedings of 5th International Business Forum on Specialty Polyolefins '95, September 20-22, Houston Tex., p. 341-352 (1995), an ethylene copolymer produced using such a metallocene catalyst has characteristics such that both a low molecular weight fraction and a high molecular weight fraction have approximately the same comonomer content, and that the comonomer content distribution is almost uniform across the molecular weight distribution of the copolymer. That is, an ethylene copolymer produced using a metallocene catalyst has a more uniform comonomer content distribution than that of an ethylene copolymer produced using a Ziegler-Natta catalyst. On the other hand, however, ethylene copolymers produced using a metallocene catalyst are still unsatisfactory with respect to desired improvements in impact resistance and ESCR properties of products of such copolymers.
Again, as was the case with Ziegler catalyst products, attempts to improve properties such as ESCR and impact resistance of products of metallocene catalysts have included their incorporation into blend compositions. A number of techniques have been proposed to prepare such blends, including a method in which two or more different ethylene copolymers having different comonomer contents are separately produced and blended by means of a kneader or a method in which an ethylene copolymer comprised of a mixture of two or more different ethylene copolymer components having different comonomer contents is produced by multi-stage polymerization (see, for example, EP 0 447 035). Further, it has also been proposed to use a method in which a mixture of two or more different types of metallocene catalysts is used to produce an ethylene copolymer comprised of a mixture of two or more different ethylene copolymer components having different comonomer contents (see, for example, U.S. Pat. Nos. 4,937,299 and 4,530,914).
However, an ethylene copolymer produced using a metallocene catalyst typically has a very narrow molecular weight distribution (Mw/Mn) of approximately 2. Therefore, when two different types of copolymers, namely a low molecular weight copolymer and a high molecular weight copolymer which are extremely different in molecular weight from each other, are produced using different metallocene catalysts, the respective amounts of copolymer chains having common molecular weights is very small in the two different copolymers, so that the compatibility between these two different copolymers is very poor.
In order to solve the above-mentioned problem, a method in which an ethylene copolymer produced using a metallocene catalyst is blended with an ethylene copolymer produced using a Ziegler-Natta catalyst has been proposed, for example, in EP 0 439 964 and EP 0 435 514. Further, a method in which an ethylene copolymer produced using a metallocene catalyst is blended with an ethylene copolymer produced by a high pressure polymerization process has been disclosed, for example, in Unexamined Japanese Patent Application Laid-Open Specification Nos. 6-207059 and 6-329848.
However there remains a requirement to produce an ethylene copolymer which not only contains few impurities such as wax, gels, but also has excellent properties, including high impact strength and excellent ESCR. There also remains a requirement to develop an ethylene copolymer which contains no impurities such as a wax, a gel while simultaneously exhibiting the above-mentioned desired comonomer content distribution, namely, in one aspect, the lower the molecular weight of a copolymer fraction, the lower the comonomer content of the copolymer fraction; and, in the other aspect, the higher the molecular weight of a copolymer fraction, the higher the comonomer content of the copolymer fraction.
There also remains a requirement to produce blend compositions comprising said ethylene copolymers which also have excellent properties, such as high impact strength and excellent ESCR properties. Finally there also remains a requirement for producing blend compositions with good compatibility between the two components and exhibiting improved uniformity and balance in properties while having low wax content and low tendency for gel formation.